Piezo motors are known. Typically, piezo motors are comprised of motors with many mm of available linear travel or any degree of rotational travel. Generally, a piezo element actuates a friction element that in turn moves a second friction element (sliding element). These piezo motors can be roughly separated into resonant and non-resonant types. Resonant type piezo motors exhibit high-speed, but are less stable at very high resolutions (nanometer to sub-nanometer range). Resonant piezo motors operate in the resonant frequency range of the piezo. Non-resonant piezo motors operate below the resonant frequency range of the piezo (and are often audible). Some of the non-resonant type piezo motors are based on the inertial or stick-slip principle and sometimes are able to achieve nano-meter resolutions.
The main problem with conventional piezo motors based on the non-resonant, stick-slip principle is that the moving part of the actuator retracts slightly during the “slip” part of the actuation cycle which results in poor constant velocity behavior, lost efficiency and a decrease of the position control of the actuator. This behavior is especially pronounced at slow velocities. Another problem with the conventional piezo motor is that the available actuation force is limited to the achievable friction of the friction element attached to the piezo element, which needs to be limited to not cause significant retraction during the slip phase of the actuator.
For example, FIG. 1A shows prior art stick-slip piezo motor 140. AC voltage source 142 provides alternating current to piezo element 141. Piezo element 141 is rigidly connected to piezo base 146. Friction element 143 is rigidly attached to piezo element 141. Friction element 143 is pressed against sliding friction element 145. During the stick phase of the cycle, piezo element 141 expands relatively slowly to the right so that friction force is not overcome and there is no slipping. During the slip phase of the cycle, piezo element 141 contracts to the left at a much faster rate to overcome the friction between friction element 143 and sliding friction element 145. The inertia of sliding friction element 145 is not overcome and there is slipping between friction element 143 and sliding friction element 145. Slipping is desired so that friction element 143 does not drag sliding friction element 145 backwards to the left. Stated differently, sliding friction element 145 presses against friction element 143 with sufficient force so that friction element 143 moves sliding friction element 145 during the stick phase of the oscillation yet also with such force so that friction element 143 does not significantly drag sliding friction element 145 backwards during the slip phase of the oscillation.
With prior art stick-slip piezo motors, there has been a problem with eliminating unwanted dragging during the slip phase. FIG. 1B shows a graphical representation of the resultant motion of a prior art stick-slip piezo motor as a function of time. As is clearly shown there is significant undesired retraction 153 during the slip phase of the cycle.
What is needed is a better stick-slip piezo motor.